1. FIELD OF THE INVENTION
This invention relates to improvements in voltage regulator circuits, and more particularly to improvements in voltage regulator circuits that have a low drop out voltage feature, and still more particularly to improvements in methods and circuits for extending the low voltage operating range of a voltage regulator circuit without interfering with the stability of the main control loop of the low voltage drop out circuit.
2. RELEVANT BACKGROUND
In many applications, it is desirable to monitor the level of a supply voltage, often to enable some specific action to be taken. For example, in many computer or electronics applications, when a supply voltage is detected that is approaching a level below which the circuit cannot properly operate, various power down routines may be initiated, for example to preserve data in a computer system, to write diagnostic data to a nonvolatile memory in an automotive system, or similar application. In the past, however, such low voltage drop out detectors have been unable to respond as rapidly as may be desired in many applications.
In a typical low voltage drop out regulator, an operational amplifier is provided that has a reference voltage applied to one of its inputs. The supply voltage is connected in a current flow path through a voltage regulating transistor, typically an MOS transistor, the gate of which being controlled by the output from the operational amplifier. A resistor is provided in the current flow path through the voltage regulating transistor to develop a voltage for application to the other input of the operational amplifier. When the voltage developed across the resistor falls below the reference voltage, the operational amplifier output changes state to turn on the voltage regulating transistor to apply the entire battery voltage (or as much of it as possible) to the output pin and load connected thereto. However, since the voltage regulating transistor has a voltage drop across it that is not insignificant, the useable voltage delivered to the load often falls faster than desired, resulting in possibly losing data that might otherwise be saved.
In efforts to correct this problem, it has been proposed to increase the size of the voltage regulating transistor. One of the problems with this solution, however, is that the size of the voltage regulating transistor limits the speed by which the supply voltage can be applied to the output pin. In normal operation, the voltage regulating transistor is held on to a degree determined by the voltage developed across the resistor so that as the voltage rises and falls, the transistor is turned on to a corresponding greater or lesser extent to provide an essentially constant voltage at the output. However, if the size of the voltage regulating transistor is increased too much, the loop stability is affected because of the higher capacitance presented by the larger sized transistor capacitance.